When you get a promotion at your job however some aspect of it "Blows" such as getting paid less, crappy office (such as it being awkwardly placed, small, and or under construction), having to work longer or all of the above. The Blow part results in people saying "This Blows" When things are bad or worse then before.

Paul: So do you like your new promotion?
John: Well I get paid less, My Office is right next to the freeway, and I have to work for 18 hours a day.
Paul: Oh that Blows
John: Yep I got a blow-motion.